2017A02 Compare and contrast O2 delivery via nasal cannulae, simple facemask and Venturi mask



·      First: describe device, fixed vs variable, max FiO2, flow rate

·      Second: reservoir, dead space, humidification

·      Third: sources of error, pros and cons


Nasal cannulae:


·  One short prong in either nostril

·  Tubing connects to cylinder or wall

·  Variable performance device

·  Gas flow rate 1-4L/min

·  FiO2 ≤ 0.4

·  Reservoir in nasopharynx but not device

·  No dead space

·  No humidification


·  Simple

·  Comfortable at low flow rate

·  No risk of rebreathing

·  Patient can eat


·  Drying and epistaxis at high flow rate or prolonged use

Factors affecting FiO2

·  ↑ Peak inspiratory flow rate (PIFR) -> ↑air entrainment -> ↓ FiO2

·  ↑VT -> ↑air entrainment

·  Hence most effective if the cause of hypoxia is hypoventilation


Face mask:


·  Mask with ~100mL volume inside

·  Holes on the size (sometimes expiratory-opening flap valves)

·  Tubing from the bottom (sometimes with reservoir bag)

·  Variable performance

·  Gas flow rate up to 10L/min

·  FiO2: up to 0.6 (up to 0.8 if reservoir bag)

·  Mask is both O2 reservoir and dead space 100mL

·  ± extra reservoir in bag ~500mL?

·  No humidification


·  Easy to use

·  Comfortable

·  Can deliver nebulized medicines e.g. salbutamol


·  Less comfortable

·  Can’t eat, can’t talk much

Factors affecting FiO2

·  As for nasal cannulae, but less affected due to reservoir


Venturi mask:


·  Well-fitting face mask

·  Exhalation pores

·  O2 supply at the bottom, larger bore tubing

·  Venturi device within the tubing (see pic)

·  100% O2 flows through a constriction

·  ? Bernoulli principle: ↑kinetic energy -> ↓potential (pressure) energy -> room air entrained due to pressure gradient

·  ? Coanda effect: air drawn in due to friction and surface tension forces (probably this)

·  Fixed performance device; specified entrainment ratio for each attachment

·  FiO2 up to 0.7 (but less accurate >0.5)

·  Mask is both O2 reservoir and dead space ~100mL

·  Humidifier present


·  Fixed performance -> can calculate PAO2, A-a gradient

·  Can be humidified


·  Uncomfortable

·  Can’t be sure of fixed performance due to leak

Factors affecting FiO2

·  ↑↑ PIFR -> air entrainment around the sides of the mask, becomes a variable performance device

·  Water -> obstruction -> ↓entrainment




Feedback welcome at ketaminenightmares@gmail.com